Transparent substrates provided with a transparent electrode over a surface thereof are in wide use for display devices, such as liquid crystal displays and electroluminescence displays, solar cells, etc. Although such transparent substrates are generally glass substrates in the prior art, substrates of polycarbonate or like resin are also known recently. Resin substrates have the characteristics of being lightweight, thin and resistance to breaking which glass substrates fail to possess, and are therefore generally used in portable electronic devices and liquid crystal display devices.
FIG. 4 is a sectional view schematically showing a conventional liquid crystal display panel wherein resin substrates are used. The liquid crystal display panel 101 comprises a first resin substrate 102 provided with a first electrode 103, a second resin substrate 111 provided with a second electrode 112, and a liquid crystal layer 104 sandwiched between the resin substrates 102, 111, with an alignment film 109 formed over each surface of the liquid crystal layer 104. The liquid crystal layer 104 is enclosed with a sealant 110.
The display panel 101 shown in FIG. 4 is in the so-called super-twisted nematic (STN) mode. Stated more specifically, stripes of electrode 103 for displaying images, characters, etc. are arranged in parallel to one another over the surface of the first substrate 102 in contact with the liquid crystal layer 104. On the other hand, similar stripes of electrodes 112 are arranged over the second substrate 111. The electrode 103 over the first substrate 102 and the electrode 112 over the second substrate 111 are so arranged that the stripes of one electrode are approximately orthogonal to those of the other electrode.
The electrode 112 which comprises an indium oxide having, for example, a tin oxide added thereto is formed over the surface of the second substrate 111 closer to the liquid crystal layer 104, and an alignment film 109 comprising, for example, a polyimide is formed over the electrode 112. Formed over the surface of the first substrate 102 in contact with the liquid crystal layer 104 are the electrode 103, which is made from an indium oxide incorporating, for example, a tin oxide, an insulating film 108 consisting mainly of SiOx and disposed over the electrode, and an alignment film 109 over the insulating film.
In producing such a liquid crystal display panel, an alignment film of polyimide resin or the like is formed over an electrically conductive film which is patterned, and the alignment film is then rubbed in a specified direction with a rubbing roll for rubbing treatment.
The insulating layer 108 is formed to prevent short-circuiting between the electrodes that would occur mainly owing to the presence of extraneous matter between the electrodes during the production of the liquid crystal display panel. This affords the display device in a greatly improved yield.
However, the use of resin substrates as the substrates of the liquid crystal display device involves the problem that the electrodes are liable to develop cracks as described in the following paragraphs (1) to (3).                (1) Resin substrates are greater than glass substrates in coefficient of thermal expansion by at least one order of magnitude, so that the difference between the substrate and the electrode in coefficient of thermal expansion becomes greater when the resin substrate is used than when the glass substrate is used. For this reason, if the resin substrate alters greatly in size in the production process of the display device owing to temperature variations involved in thermal load or in the actual environment for use, the electrode will fail in following the alteration of the resin substrate, so that a great stress occurs in the electrode. The electrode is very thin, low in mechanical strength and therefore susceptible to cracking due to the stress produced. Further various films, such as the insulating film and alignment film, formed over the electrode in superposed layers further increase the stress in the electrode, which in turn becomes more susceptible to cracking.        (2) Resin substrates are smaller than glass substrates in Young's modulus by at least one order of magnitude and have high flexibility. Resin substrates are therefore more resistant to breaking by external forces, but are deformable. When subjected to an external force, the resin substrate accordingly deforms greatly, rendering the electrode liable to cracking.        (3) Resins are generally hygroscopic, and the resin substrate deforms or expands when absorbing moisture, possibly permitting the electrode to develop cracks.        
Thus, the resin substrate used for forming the electrode thereon renders the electrode susceptible to cracking. Especially in the case where a resin substrate is used for each of the two substrates for providing the liquid crystal display panel, affixing the substrates to each other restrains the substrates from deforming, with the result that the problem of cracking becomes more pronounced.
An object of the present invention, which has been accomplished to overcome the foregoing problem, is to provide a liquid crystal display panel wherein the electrodes are effectively prevented from breaking due to cracking, and a process for producing the panel.